This invention relates generally to a CATV system in which a central facility is connected through cables to a plurality of terminal units in a manner such that two-way data communication is effected therebetween. More particularly, it is related to a data communication method in such a CATV system in which for each channel an exclusive downstream data signal is inserted and a responsive up-stream data signal from each terminal unit is transmitted through a channel in correspondence to the down-stream channel used. In this manner data communications can be carried out separately through the channels, and frequency dividers are used to readily determine the carrier frequencies of the up-stream data signals.
A typical CATV system will now be described with reference to FIG. 1. The CATV system includes a single base or center 1 and terminal units 28 which are provided, for example, in several tens of thousands of subscriber's homes. The center 1 is connected through coaxial cables to the terminal units 28. A main cable 3 extends from the center 1, and main cable amplifier 4 and branching units 5 are provided at predetermined positions along the main cable 3. A plurality of branch cables 6 extend from each branching unit 5. Extension amplifiers 7 and tap-off units 8 are provided at predetermined points along the branch cables 6. Each tap-off unit 8 is connected to branch lines 9 which extend to the terminal units 28 within the individual homes 2. Each terminal unit 28 includes a main unit 10, a television set 11 and a control unit 12. The branch lines 9 is connected to the main unit 10, which is in turn connected to the television set 11 and to the control unit 12. As is apparent from the above description, the main cable extending from the center is branched into a first plurality of branch cables, which are in turn further branched into a second plurality of branch lines which are connected to the terminal units 28.
A signal receiving antenna 13 is installed outside the center 1. The antenna 13 is connected to a demodulator 15 in a source group 14 of the center 1. The source 14 further includes a video disc player 16, a video tape recorder 17 and a studio 18. The output signals of the source group 14 are applied to a modulation output section 19 composed of two systems. One of the two systems includes an IF modulator circuit 20, a scramble circuit 21, and an up-converter circuit 22, and the other system includes an IF modulator circuit 23 and an up-converter circuit 24. The outputs of the up-converters 22 and 24 are connected to the main cable 3, to which a data transceiver 25 is connected for data communication between the center 1 and each main unit 10. The data transceiver 25 is connected to a computer 26 which is connected to a peripheral unit 27 such as a printer or a display unit.
The operation of the CATV system thus constructed will now be described.
First, the power switch of the television set 11 is turned on. With the television set 11 set to a particular unused channel, the control unit 12 is operated so that the frequency of a desired channel to be received is set to that of the unused channel. The channels which can be selected by the control unit 12 as described above can be classified into three groups of channels, to wit (A) "retransmission programs" in which television signal from local stations are received without modification; (B) "independent programs" (free of charge); and (C) "chargeable" (pay) programs. Each group is allocated ten channels, and therefore any of the thirty channels can be selected by operating the control unit 12.